Apparatus for comminuting concretions is previously known, e.g. from DE, Al, 3220751.
In this type of apparatus it is important to achieve good comminuting effect on the concretions or calculi (kidney or gall stones) while the pain caused to the patient is sufficiently low so that anesthetics are not required. Analgestic and optional local anesthesia, e.g. EMLA cream (cylocain) on the skin where the wave inpinges should be sufficient for the procedure. In this way the cost of anesthetics is reduced.
It has been found that the effective frequency for adequate therapy is in the range 0.3-1 MHz. For the frequency of 1 MHz and a sound propagation velocity of 3000 m/s in kidney and gall calculi there is obtained a wavelength of .lambda.=3 mm at these sites. Disintegration in calculi is achieved in the range 1/4-1/2.lambda., i e. fragments of the size 0.75-1.5 mm, these being desired sizes. Of course, calculi are not homogeneous, i.e. disintegration can occur to an important degree due to inherent weakness bands. Fragments with the given sizes can subsequently be passed without causing further trouble.
Lower frequencies do not give satisfactory therapy, e.g. for 100 kHz there is magnitude of disintegration in the range of 0.75-1.5 cm, which gives fragments which are much too large to be passed easily.
Such low frequencies are not focused particularly well in the present size of the reflector and will thus pass into the body as a badly focused wave. They involve large displacements causing pain to the patient and by sudden jerks in the heart area they can cause the risk of cardiac arhythmia of different kinds, such as auricular fibrillation and flutter.
It is therefore an object of the invention to minimize the energy transmission at such low frequencies and concentrate it to the frequency range 0.3-1 MHz. Substantially higher frequencies than 1 MHz are attenuated too heavily in the body tissue for having a good effect on calculi.